#include <iostream>
#include <string>

using ll = long long;

auto &is = std::cin;
auto &os = std::cout;

// const ll max_n = 10+2;

ll _;
std::string s;

void dfs(const ll start, const ll end)noexcept{
    if(start < end-1){
        const ll mid {(start+end)/2};
        dfs(start,mid);
        dfs(mid,end);
    }
    bool is_not_B = false,is_not_I = false;
    for(ll i{start};i!=end;i++){
        if(s[i]!='0')is_not_B=true;
        if(s[i]!='1')is_not_I=true;
    }
    if(!is_not_B){
        os<<'B';
    }else if(!is_not_I){
        os<<'I';
    }else{
        os<<'F';
    }
}

int main(){
    is>>_;
    is>>s;

    dfs(0,s.size());
}